Styrene butadiene rubber (SBR), having a structural formula as shown in FIG. 1, has the highest volume production in this country of any synthetic rubber. It is used extensively in the manufacture of automobile tires and tire-related products, as well as other products, including but not limited to sporting goods, hoses, footwear, flooring, wire and cable, raincoats, and rain boots. There is a significant need for effective recycling methods for SBR. The number of spent automobile tires discarded annually is estimated in the hundreds of millions. Hundreds of millions of tires from used automobiles are discarded annually, while the number of new automobile tires put into service each year, from new car production only, is estimated to exceed three hundred million.
SBR is synthesized by a process known as emulsion polymerization, which is known in the art. Polymerization of the styrene and butadiene copolymers is initialized in the aqueous phase to form a latex material at an approximate ratio of butadiene to styrene of about 3:1. The synthesized polymer then undergoes vulcanization to form sulfur cross-links, represented in FIG. 2, which help to impart upon the styrene butadiene base polymer the properties that are generally associated with rubber. After vulcanization, the rubber is compounded with additives which are also known in the art, to enhance properties of the rubber such as tensile strength, elongation resilience, hardness, and abrasion resistance. Table I presents typical compositions of SBR used for tire tread, in which PHR refers to parts per 100 parts of SBR base polymer.
TABLE IEXAMPLE COMPOSITION OF SBRIngredientsPHRSBR Rubber100Carbon Black60Highly aromatic oil20Antiozonant1Antioxidant1.5Zinc Oxide3Stearic Acid1Retarder1Sulfur1.75Primary Accelerator1Secondary Accelerator0.2
SBR, and virtually all other vulcanized rubbers, are distinguishable from thermoplastic polymers such as polyethylene or polypropylene in that thermoplastic polymers can be melted and reused in other products, but vulcanized rubber cannot because of the interconnected network of polymer chains and sulfur cross-links formed during vulcanization. Consequently, SBR recycling is largely limited to macroscopic, non-chemical processing of the material so it can be used in other products, such as floor mats, blasting mats, traffic cone bases or soft pavement used in athletic tracks. However, these uses only account for less than 10% of all tires discarded annually. While there are still other isolated uses for spent tires, the substantial majority of tires consumed is sent to landfills, which are not an ideal solution for such large-scale disposal. Unquestionably, with the large and continuously growing market for SBR, and the inherent challenges associated with its disposal, there is a significant need for improved methods for tire recycling.
The properties of SBR, natural rubber, and neoprene rubber are well-known in the drinking water distribution industry, because such materials are used extensively, e.g., for gaskets and hoses at water treatment facilities. In view of various environmental standards and regulations, limiting the use of free chlorine for treating and disinfecting water, the use of chloramine compounds for these purposes has increased. Chloramine forms by reacting ammonia and chlorine, and is present as monochloramine (NH2Cl), dichloramine (NHCl2), and nitrogen trichloride (NCl3).
A 2007 study published by the American Water Works Association (AWWA) reported on the degradation of SBR by monochloramine. In general, SBR was found to have moderate sensitivity to monochloramine. The findings of experiments performed at the highest temperatures and concentrations used in the study, i.e., 70° C. and 60 parts per million (ppm), respectively, were consistent with the SBR losing its sulfur cross-links, which provides SBR with its favorable mechanical properties but also restricts its recyclability. Thus, monochloramine's effect on SBR in the study suggested possible opportunities from a recycling perspective, although the study was carried out over 30 days and the level of degradation was incomplete. Accordingly, given the large scale recycling needs for automobile tires and other SBR-containing products, and recognizing the long time constraints associated with the AWWA study, there is a significant need for more robust, efficient and cost-effective methods of degrading and recycling SBR in shorter periods of time.
Further, other challenges are inherent to SBR, with respect to exposing this material to monochloramine with the objective of breaking the sulfur cross-linking bonds. For example, SBR is an amorphous polymer having a tightly packed sulfur cross-linked matrix of styrene-butadiene polymer strands, which impairs the diffusion of fluids through such a dense network. In this context, diffusion refers to the ability of a chloramine compound to penetrate the rubber matrix of SBR or like materials and react with the sulfur cross-links. Reaction kinetics (the rate at which reactants form products) governs the reaction between these compounds and the sulfur cross-links once the diffusion has occurred. Accordingly, the degradation of SBR and like materials is limited by the rate of diffusion, and the rate of diffusion is affected by the concentration of monochloramine at any given point in time. Consequently, there is a need to supply a steady and consistent concentration of monochloramine to the SBR, while overcoming the challenge created by the fact that monochloramine is thermodynamically unstable and decomposes spontaneously.